Although half of the cases of type 1 diabetes occur in adulthood, there are very limited data on the incidence of type 1 diabetes in the U.S. adult population. This is in part due to the difficulty in distinguishing between type 1 and type 2 diabetes in this segment of the population and performing complete case ascertainment in fragmented health care systems. This study fills these gaps by assessing the performance in terms of timeliness, accuracy, and costs of a surveillance system for type 1 diabetes in the young adult population.